In offshore oil exploration, it is often necessary to protect sub-sea equipment from damage from anchors, nets, icebergs and other marine hazards. Equipment protruding from the sea floor in high traffic areas such as shipping lanes is in danger of damaging or being damaged by passing vessels. Environmental damage can result from oil spills caused by damage to the wellhead equipment by such hazards. To date, sub-sea drilling equipment such as blowout preventers, and well completion equipment, such as "Christmas trees" have been located at the sea floor level, leaving them exposed to such damage. In some areas the subsea production equipment has been covered with a structure resembling a bee-hive, although drilling equipment has been unprotected.
The problem is particularly acute in Arctic waters where the ice pack and icebergs scour the ocean floor with great force. Consequently, excavations in the ocean floor, called glory holes, have been used to keep equipment below the ice keel interaction zone or to protect the wellhead from damage from anchors, fishing equipment, or shipping. The glory holes are excavated by dredging a depression of sufficient size so that the slopes are stable. These holes can be expensive to dredge where difficult soil conditions are encountered or the water is deep. Also the protection provided against scouring by ice is imperfect, as the ice pushes a pile of rubble ahead of it as it scours the ocean floor, and little protection from anchors, fishing nets and the like is provided.
Improvements to the conventional uncased glory hole have been proposed in a paper entitled "Gloryhole Tool: Design, Fabrication and Operation" by H. R. Stewart, I. MacGregor, T. V. Goudoever and R. E. Isted presented at the 1986 Arctic Offshore Technology Conference, Calgary, Alberta, a paper entitled "Beaufort Sea Cased Gloryhole Drilling" by D. Gilbert, I. MacGregor, and K. J. Vargas presented at the Eighth International Conference on Offshore Mechanics and Arctic Engineering Mar. 19-23, 1989 and in a paper presented at the 21st Annual Offshore Technology Conference in Houston, Texas, May 1-4, 1989, entitled "Drilling and Installing Large-Diameter Caissons for Wellhead Protection" by B. W. Meadows and D. C. Gilbert. According to this method, a glory hole tool is used to drill a cylindrical glory hole 7 to 14 meters in diameter. The glory holes are typically 8 meters in diameter for exploration wells and 13 meters in diameter for production wells. Such holes will be 10 to 20 meters deep. A cylindrical metal casing is left in the glory hole when the tool is retracted. The glory hole can be drilled from a drilling rig or a support vessel or barge.
As described in the papers noted above, the glory hole excavation system uses a bit consisting of a rigid and a rotating section. The rigid section includes the main ratchet drive assembly, a structural roof section which incorporates retractable pins for disengaging from the casing, and a peripheral skirt which acts as a foundation for the casing. The rotating component has three unequal length cutter arms with separately rotating cutting discs and a central penetration spear. Retractable spades extend horizontally from the drilling end to prevent rotation. The cylindrical casing system consists of a leading skirt followed by a corrugated metal pipe which is supplied in sections bolted to the skirt and each other. The top section is sacrificial so that the upper cylindrical section of the casing can shear off if scoured by ice without dislodging the rest of the casing. The casing is carried with the bit and after completion of the glory hole excavation it is released from the bit to remain in the glory hole.
Various patents are of interest in the area of cased sub-sea wells or glory holes. Canadian Patent no. 995,583 issued Aug. 24, 1976 discloses a sub-sea well structure in which the production equipment is protected by a cased cavity below the ocean floor. U.S. Pat. No. 4,744,698 issued May 17, 1988 discloses a method of installing marine silos or caissons for forming a protective chamber for sub-sea wells.
Cased glory holes provide a number of advantages. They are faster and less expensive to make, as less material is moved. Better protection against ice scouring, anchors, fishing nets and trawling boards is provided. However certain problems remain. The cased glory holes may fill with silt, particularly where adjacent drilling operations are disturbing the sea bed. Over time the accumulation of silt can interfere with the operation and maintenance of equipment in the glory hole. Also very large diameter glory holes provide less protection against anchors, nets and the like. Currently, human divers are used to make necessary connections and carry out operations in the glory hole, which is risky and expensive, particularly in deep water.